Electro-acoustic transducer



July 7, 1959 5. J. ZUERKER ET AL 9 ELECTRO-ACOUSTIC TRANSDUCER FiledOct. 19, 1955 F\G.l.

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INVENTORSI SIEGFRIED J. ZUERKER, BENJAMIN J. SAGGESE,

, THEIR ATTORNEY.

United States Patent ELECTRO-ACOUSTIC TRANSDUCER Siegfried J. Zuerker,Pennellville, and Benjamin J. Saggese, Liverpool, N.Y., assignors toGeneral Electric Company, a corporation of New York Application October19, 1955, Serial No. 541,436

2 Claims. (Cl. 317-201) This invention relates to an improved structurefor the magnetic component of a transducer device.

It is an object of this invention to provide a novel magnetic structurefor a transducer, such as a loudspeaker which permits fabrication at areduced cost.

It is another object of this invention to provide a novel magneticstructure for loudspeakers, which structure has a configurationpermitting it to be molded out of a powdered magnetic material.

It is still another object of this invention to provide a novel magneticstructure for loudspeakers, wherein the magnetic structure may beassembled to the rest of the speaker assembly by the use of a portion ofthe magnetic structure itself so that the cost of assembly is reduced.

In the prior art wherein one component of magnetic material has beensoldered to another, it has been found necessary to first coat thecomponents with a suitable base before applying the solder. This methodof bonding results in a relatively thick effective air gap between thetwo members and a consequent increase of reluctance in the circuit.Therefore, it is a further object of this invention to provide a novelmethod of bonding magnetic materials so that the effective air gapbetween the materials is minimized.

It is still a further object of this invention to provide a novel meansand method of bonding components of magnetic structures to one anotherso that the cost of assembly of these components is reduced and thesensitivity of the structures are increased.

Briefly, the objects of this invention are achieved in one form by theprovision of a unitary structure forming a portion of the magneticcircuit of a loudspeaker, which unitary structure has provided thereonan integral means supporting the magnetic structure on the speakerbasket. A second portion completes the magnetic circuit and is assembledto the remainder of the magnetic structure by the use of a bondingmaterial which reduces the equivalent air gap normally existing betweenthe second portion and the rest of the magnetic structure and therebycuts down the consequent flux loss and yields a more sensitiveinstrument.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention itself, however, both as to its structure and method ofassembly, will best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in which:

Figure 1 is a cross-sectional side view of a speaker embodying ourinvention; and

Figure 2 is a rear view of the speaker illustrated in Figure 1.

Referring now to Figure l of the drawing, it may be seen that thespeaker is constituted by the supporting frame or basket 1. At the outerperiphery of the basket a flange 2 supports the outer edge of afrusto-conical diaphragm 3. If desired, a spacing or securing ring, suchice as shown at 4, may be provided at the outer edge of the cone. At thecenter of the cone 3 there is provided a substantially tubular voicecoil form 5 which is coupled to and drives the speaker cone 3 in themanner well known in the art. Also, in the manner well known in the art,we provide a flexible spider 6, which is secured at its outer peripheryto the basket as shown at 7. A moving or voice coil 8 is wound on thevoice coil form 5 and drives the diaphragm 3 in accordance with thevarying electrical signals representing sound intelligence impressedthereon. The construction described is substantially that constructionwell known in the art and constitutes no part of our invention.

In accordance with our invention, we provide a permanent magnet 9 whichmay have a cylindrical configuration aud is disposed, as illustrated inthe drawing, just to the rear of the basket 1. The magnet may, ifdesired, be constituted by an electro-magnet or may be a permanentmagnet formed from the material known by the trade name Alnico V whichis commonly used in the art for this purpose. A pole plate 10 ofmagnetic ma terial may be secured by any suitable means to the front ofthe magnet 9 or may be held by the action of the magnet itself andprovides a path to carry magnetic flux to a region surrounding the voicecoil 8. Alternatively the pole plate 10 may be eliminated and the magnet9 extended to the region occupied by the pole plate. A cup or yokestructure 11, which is formed as a tube and which may have a circularcross-section, encompasses the magnet 9 and also constitutes a portionof the magnetic circuit of the device. The remainder of the magneticcircuit is completed by the bottom plate 12 which may be formed from asoft steel or other magnetic material and is secured to the rear end ofthe magnet 9 and engages the inner surface of the larger inner diameterend of the cup. A forward portion of the cup 11 is provided with aportion extending radially inward as shown at 13 and presents a smallerinner diameter. In combination with the pole plate 10, the portion 13forms an air gap 14 in which the voice coil 8 is situated, whereby theelectrical conductors constituting this coil may be traversed by themagnetic fiux emanating from the magnet 9. The extent of the inwardlydirected portion is selected in accordance with the principle well knownin the art so that the required spacing between the outer periphery ofthe magnet 9 and the inner wall of the cup 11 is at an optimum tomaintain fringe flux losses between these two members at a minimum. Theforward end of the portion 13 is provided with a means whereby the cupmay be secured to the basket 1 by positioning extension 13, in anopening in the rear portion 15 of the basket 1 and peening it over toengage the frame in the manner indicated by reference numeral 16.

The construction of the magnetic portion of the speaker is such that theyoke or cup 13 may be molded using any suitable material capable ofcarrying magnetic flux. Thus, it is possible to utilize a powdered ironmaterial for this portion of the assembly which may be fabricated bymolding so as to considerably reduce the cost of this portion of thedevice. This particular advantage is gained by providing that theforward reduced inner diameter portion 13 of the cup constitutes theelement completing the air gap, necessary for the operation of the voicecoil.

As pointed out above, the magnetic circuit is completed by the bottomplate 12. The bottom plate, in addition to completing the magneticcircuit, also functions to make possible the use of material having arelatively low saturation value, and consequently relatively inexpensivematerials, as it reduces the flux density by providing a path of everincreasing cross-section from its center to its periphery where itengages the cup 11. Thus, a powdered iron material is capable oftransmitting the required flux without incurring high losses due to highflux density.

A further feature of our invention is the manner in which the bottomplate 12 and the pole plate are secured to the permanent magnet 9. Ithas been the custom in the prior art to solder such assemblies. Thesoldering operation has been carried out by coating the members with abase such as cadmium and by depositing pellets of solder and fluxbetween the surfaces of the two members after the coating. Heat is thenapplied and the solder disperses over the two surfaces. However, thestructure which thereby results is subject to the disadvantage of whatamounts to a fairly large air gap exists between the two members 9 and12. This air gap may be on the order of 0.002 of an inch and theresulting flux losses serve to reduce the sensitivity of the device.

In accordance with our invention, we provide that the opposing surfacesof the pole plate 10, bottom plate 12 and the magnet 9 shall each becleaned by the application of a suitable cleansing agent and then coatedwith a relatively thin coat of tin-zinc, It has been found that atinzinc composition of from 73 %83% tin and from 27%- 17% zinc, byweight, yields a bond of sufiicient strength to hold the memberstogether. Within this range, a composition of 78% tin and 22% Zinc, byweight, appears to yield a bond of optimum strength characteristics. Wehave found that electroplating is the most effective method for forminga coating of tin-zinc on the surfaces to be joined. The members 9, 10,and 12 are then subjected to heat. The tin-zinc forms, upon cooling, astrong bond to hold the members 9, 10 and 12 together. The equivalentair gap between these members is far less than that obtained by othermethods such as noted above and the consequent losses are avoided. Theair gaps, obtainable by the use of this method, are on the order of0.0002 of an inch and an overall increase in speaker sensitivity isobtained. Such an advantage permits either the construction of a moresensitive speaker or, if sensitivity comparable to competing speakers isdesired, a smaller magnet with its consequent cost reducing feature maybe used. We have found that if a further decrease in the equivalent airgap is de sired, pressure may be exerted on the elements 9, 10 and 12 toforce out Whatever bubbles or undue thickness of plating material may bepresent.

The bonding technique, described above, has particular utility informing structures constituting a magnetic circuit wherein it isdesirable to avoid flux losses. However, it is not limited in itsapplication to loudspeaker magnetic structures, but obviously hasgeneral utility in fabricating magnetic devices of diversecharacteristics.

While the present invention is described by reference to a particularembodiment thereof, it will be understood that numerous modificationsmay be made by those skilled in the art without actually departing fromthe invention. We therefore, aim in the appended claims to cover allsuch variations as come within the true spirit and scope of theforegoing disclosure.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A magnetic assembly comprising a cup of one magnetic material, oneend of said cup defining a first opening, the other end of said cupdefining a second opening, a bottom plate of another magnetic materialmounted on said cup so as to close said second opening, a magnet mountedon said bottom plate inside said cup and extending in the direction ofsaid first opening, the magnetic material of said cup being such that itwould be saturated by flux emanating from an end of said magnet, and thematerial of said bottom plate being such that it can carry the fluxproduced at the end of said magnet without saturating.

2. A magnetic structure as defined in claim 1 wherein the cup is made ofmolded magnetic material.

References Cited in the file of this patent UNITED STATES PATENTS2,047,361 Dijksterhuis July 14, 1936 2,110,200 Clarke Mar. 8, 19382,397,400 Barwich Mar. 26, 1946 2,398,449 Ranci Apr. 16, 1946 2,705,263Schoenberg Mar. 29, 1955 2,786,983 Hill Mar. 26, 1957 FOREIGN PATENTS586,500 Great Britain Mar. 20, 1947 1,052,050 France Jan. 20, 1954

